Devices for preparing bituminous coated materials for road surfaces, using new aggregates, liquid bitumen, used bituminous mixes which have been recycled and powdery products, are known. These devices generally consist of a cylindrical drum of large dimensions, mounted on a platform for rotation about its axis and slightly inclined relative to the horizontal plane. The new aggregates and powdery products are introduced into the drum at one of its ends and the recycled mix in granular form via a recycling ring surrounding the drum in an intermediate zone between its two ends.
A burner penetrates inside the drum at one of its ends and enables hot gases to be circulated inside the drum, these gases ensuring drying and heating of the materials circulating inside the drum.
Such a drum carries out both drying and heating of the cold and moist aggregates entering into the drum, heating of the recycled mix and mixing of the new aggregates and the recycled mix in contact with liquid bitumen supplied into the drum via an injection pipe.
The internal wall of the drum is equipped with vanes of varying shapes depending on the zones of the drum so as to ensure, as a result of rotation of the drum, transportion, stirring and/or lifting of the materials circulating inside the drum.
Compared to former methods where the aggregates were dried and heated inside a rotating drier and the liquid bitumen mixed inside a separate mixer having a stationary housing, the integration of the drying, heating and mixing functions inside the same drum has led to a certain simplification of the procedures and the materials involved. However, drying/mixing drums have the drawback that they give rise to the presence, inside the same enclosure, of a flame, very hot gases and liquid bitumen. This results in the bitumen vapor being entrained by the hot gases circulating inside the drum, which in turn leads to rapid clogging of the bag filters used to extract the dust from the gases leaving the drum and to the discharge of harmful vapors into the atmosphere. These drawbacks are particularly marked in the case of parallel-flow drying/mixing drums, i.e., drying/mixing drums where the hot gases circulate inside the drum in the same direction as the solid matter.
It has therefore been proposed to return to the old idea of coating materials in which drying and heating of the aggregates on the one hand and mixing with liquid bitumen on the other hand are carried out in different enclosures.
However, this technique is not suited to the preparation of mixes using new aggregates and a certain proportion of recycled bituminous mix.
A new type of equipment, which may be referred to by the name "drier/recycler", and which performs both drying and heating of the new aggregates, reheating and melting of the recycled mix in granular form and mixing of the said mix with the dried and heated new aggregates, has therefore been devised. The intermediate bituminous product obtained at the outlet of the drier/recycler may be introduced into a mixer into which there is injected liquid bitumen which ensures the final coating of the granular products pre-coated inside the drier/recycler.
The drier/recyclers known and used to date are, however, designed in such a way that very hot gases come into contact with bituminous products, and in particular with the recycled mix, just before they leave the drum. In these known drying/recycling devices, the hot gases therefore entrain bitumen vapors which reappear in the atmosphere in the form of blue fumes well known to the operators of road surfacing plants.